Engines may be configured with exhaust gas recirculation (EGR) systems to divert at least some exhaust gas from an engine exhaust manifold to an engine intake manifold. By providing a desired engine dilution, such systems reduce engine knock, throttling losses, in-cylinder heat losses, as well as nitrogen oxide (NOx) emissions. As a result, fuel economy is improved, at part throttle loads and at higher load levels such as during engine boost. As an example, by recirculating a portion of the engine's exhaust back to the engine cylinders, the oxygen in the incoming air stream is diluted and gases inert to combustion act as absorbents of combustion heat to reduce peak in-cylinder temperatures. Because NOx forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature, the lower combustion chamber temperatures caused by EGR reduces the amount of NOx generated from combustion. Engines have also been configured with a sole cylinder (or cylinder group) that is dedicated for providing external EGR to other engine cylinders. Therein, all of the exhaust from the dedicated cylinder group is recirculated to an intake manifold. As such, this allows a substantially fixed amount of EGR to be provided to engine cylinders at most operating conditions. By adjusting the fueling of the dedicated EGR (DEGR) cylinder group (e.g., to run rich), the EGR composition can be varied to include species such as Hydrogen and CO which improve the EGR tolerance of the engine, resulting in fuel economy benefits.
When one or more cylinders are dedicated to providing EGR, under standard fueling and controls, the EGR fraction in the charge flow is simply the ratio of the number of EGR cylinders to the total number of cylinders. As an example, an engine comprising one DEGR cylinder out of a total of four cylinders will operate at 25% EGR if all cylinders are operated similarly. While such an arrangement simplifies engine operation in terms of controls, hardware devices, etc., the simplified operation results in a general lack of control over the system. For example, a key disadvantage is the inability to reduce EGR rate at light loads, where combustion stability is a constraint. Another example where lack of control may be disadvantageous is during transient conditions where the pressure of the charge flow in the intake manifold can change more rapidly than the pressure of the exhaust in the exhaust manifold of the dedicated EGR cylinder(s), such as when the driver tips out of the pedal causing the throttle to close quickly. In such an example, the EGR fraction provided may increase significantly over the expected or desired EGR fraction. Deviations from expected or desired EGR fractions may lead to undesired operating conditions, such as cylinder misfire, and combustion instability. As such, it is desirable to enable control over dedicated EGR during light loads and transient conditions, without substantially increasing costs.
In another example, a disadvantage of operating an engine with one or more DEGR cylinders is the inability to regulate the amount of EGR during engine starting conditions. For example, during the early stages of an engine cold start, the temperature of the intake passages and the combustion chambers of the engine may inhibit the proper vaporization of fuel. As a result, unburned fuel vapor may be delivered to an exhaust catalyst of an emission control device. During such cold start conditions, the catalyst material in the emission control device may not be at a sufficient temperature (e.g., light-off temperature) in order to sufficiently process the undesired, uncombusted by-products of combustion, and an increase in undesired tailpipe emissions may thus result. In an engine configured with one or more DEGR cylinders, such issues may be exacerbated if fuel injection is provided to the one or more DEGR cylinders during a cold start event. For example, combustion stability may be further compromised, leading to the possibility of delayed engine start, engine stalls or hesitations, and excessive undesired emissions.
One solution to reducing emissions during a cold start is the use of an Electrically Heated Catalyst (EHC). As such, the heated catalyst is better able to process undesired by-products of combustion. However, use of the EHC adds additional cost, complexity, and importantly, requires a delay prior to engine starting to allow the EHC to preheat the catalyst. Accordingly, improved systems and methods for engine cold start operations, particularly in an engine comprising one or more DEGR cylinders, are desired.
U.S. Pat. No. 8,996,281 teaches a method of using dedicated EGR to decrease the light-off time required for a catalytic exhaust aftertreatment device. If the engine is determined to be in a cold start condition, a valve is activated to direct dedicated EGR to a bypass line, the bypass line configured to route EGR gas to a point directly upstream from the aftertreatment device, and directly downstream of an exhaust turbine. In such an example, the DEGR is optimized for temperature rise, which includes operating the DEGR cylinder(s) at a rich air-fuel ratio so that the EGR has high concentrations of H2 (hydrogen) and CO (carbon monoxide). Furthermore, a secondary air valve is commanded open during cold start conditions, to provide O2 to the bypass line. As a result, the H2 in the DEGR oxidizes with O2 in the exhaust, and the H2-enriched DEGR is hot and not in contact with the large thermal sink of the exhaust turbine. As such, catalyst light-off times may be improved as compared to cold start events without DEGR heating. However, the inventors herein have recognized potential issues with such a method. For example, costs and complexity of the engine system may be increased as a result of additional bypass line(s), bypass valve(s), and air valve(s) to control O2 in the bypass line(s).
US Patent Application US 20160025021 similarly teaches flowing exhaust from a DEGR cylinder to each of an exhaust catalyst via a bypass passage and an engine intake via an EGR passage, and adjusting a relative flow through the passages via the bypass valve, the adjusting responsive to catalyst temperature. As such, the bypass valve may comprise a continuously variable bypass valve that allows a portion of the exhaust gas to be metered to an exhaust catalyst via the bypass passage, while the remaining portion of the exhaust gas may continue to be recirculated to the engine intake via the EGR passage. In one example, US 20160025021 teaches that during conditions when catalyst temperature is below a threshold, such as during a cold start condition or after an extended operation at light load, the bypass valve may be adjusted to increase exhaust flow through the bypass passage while correspondingly decreasing exhaust flow through the EGR passage. In addition, the DEGR cylinder may be enriched to provide a H2, CO, and hydrocarbon-rich exhaust stream at the exhaust catalyst, where the degree of richness may be adjusted based on the heat flux required to bring the exhaust catalyst to or above a threshold temperature. However, the inventors herein have additionally recognized potential issues with such a method, namely that the use of a continuously variable valve, in addition to a bypass passage, may increase the costs and complexity of the engine system.
Thus, the inventors herein have developed systems and methods to at least partially address the above issues. In one example, a method is provided comprising, coupling an exhaust from one or more cylinders (DEGR cylinders) of a multiple cylinder combustion engine to an intake manifold of the engine, and in a first condition, including a cold start and warm-up of the engine, shutting off fuel and spark to the DEGR cylinders while maintaining intake and exhaust valves activated on the DEGR cylinders (e.g. remaining cylinders), and resuming fueling and spark, and maintaining activated intake and exhaust valves on the DEGR cylinders in a second condition.
As one example, the first condition includes temperature of one or more catalysts coupled to exhaust from the non-dedicated EGR cylinders being below a predetermined threshold temperature needed for catalytic activity, and the second condition includes an indication that temperature of one or more catalysts coupled to exhaust from the non-dedicated EGR cylinders has reached the predetermined threshold temperature. Furthermore, one example includes retarding ignition of the engine during starting of the engine under the first set of operating conditions. In this way, by shutting off fuel and spark to the DEGR cylinders while maintaining activated the intake and exhaust valves on the DEGR cylinders, air may be routed to the intake of the non-dedicated EGR cylinders instead of exhaust resulting in exhaust gases lean of stoichiometry. By retarding ignition to the non-dedicated EGR cylinders in addition to enleaning exhaust gases from the non-dedicated EGR cylinders, time needed to elevate temperature of the one or more exhaust catalysts to the predetermined threshold temperature needed for catalytic activity may be reduced. As such, combustion stability issues during engine cold starts may be avoided by stopping fueling and spark to the DEGR cylinder, and by using the DEGR cylinder as an “air pump” to rapidly heat the catalyst, undesired emissions may be avoided.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.